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Measuring temperature with acoustic waves

No batteries and no wires: is this the perfect sensor?

Temperature has been measured in so many different ways over the past hundred years, you might be forgiven for thinking industry has run out of new ways to do it.

But a startup company located on a palm tree-lined boulevard near Cannes, called SENSeOR, (www.senseor.com) challenges this assumption. It has launched three new sensing solutions, all based on surface acoustic wave (SAW) technology.

SAW devices certainly aren’t new: they have been used in the telecommunications industry as bandpass filters since the end of the war, and are now beginning to find their way into the medical and automotive markets as chemical sensors and torque and pressure sensors.

These sensors use piezoelectricity to apply an oscillating electric field to create a mechanical wave, which propagates across a surface and is then converted back to an electric field for measurement.

As the wave travels, it is sensitive to many different physical parameters—so sensitive, in fact, that SAWs used in telecommunications have to be hermetically sealed to keep their operation stable.

But this susceptibility to perturbations in the environment is what makes the SAW device a potentially responsive detector. And because most of SAW technology is based on the piezoelectric effect, very low power is required to animate it; so low, in fact, that devices can be built that are activitated entirely by radio energy, without recourse to batteries or wires.

SENSeOR's adaptation

The temperature and humidity dependence is, in fact, what SENSeOR is exploiting. The company is doing this by building the sensors into rugged packages suitable for industrial environments. Each measurement solution consists of a reader unit operating in the ISM band, at 434 MHz, and a totally passive sensor mounted a couple of metres away.

The electromagnetic wave sent by the reader unit is received and converted into an acoustic wave by means of a transducer located at the surface of a piezoelectric crystal (typically quartz).

Temperature variations related to the nearby environment trigger a modification of the acoustic wave’s physical properties—in particular, its velocity, or delay length—as it skims along the surface of the device. The modified acoustic wave is then transformed back into an electromagnetic wave to be retrieved by the reader unit.

SENSeOR says it can measure the delay or corresponding phase, the attenuation, and the frequency of the sensor’s electrical response.

To compensate for unwanted effects from other physical parameter variations, there is an independent reference sensor on the same chip which provides a differential measurement.

Currently the sensors measure to 175°C with a precision of ±2°C. The company is working on extending this to 350°C. There are three products in the initial launch, one a free standing 5x5 mm package and two thermowell temperature probes.

Highly stable

SENSeOR says that, given the right design, it can attain a very large measurement range at very high sensitivity and high stability, to within one in a million parts per year. The rugged construction is unaffected by strong electromagnetic fields or radiation, and typically has a very long lifetime.

Andreas Jagtøyen, at Kongsberg Maritime AS in Norway, who has industrialised a SAW-based in-engine bearing temperature sensor for vessel motors, says “We have installed thousands of systems working in the toughest conditions, and none has ever failed. We will use this technology for other break-through applications.”

François Gégot, Director of Business Development at SENSeOR, notes “We continuously do our best to understand in details the way we can help maintenance, production and process engineers. Our latest product innovations aim to match their expectations such as high temperature measurements without costly and unmanageable cables, together with robustness and environmentally friendly concerns.

“Temperature measurements inside rotating machines such as engines, turbines, and pumps, as well as tanks, ovens, buried pipelines or even inside materials like concrete with sensors incorporated inside, become easy and accessible.”

SENSeOR’s wireless ‘reader’ sends power to the sensor and interrogates it for temperature data